This invention relates to polymer compositions of the type adapted to be applied to fibrous substrates so as to improve their physical properties, notably the wet and dry strength of paper substrates.
The invention particularly relates to polymer compositions that are capable of curing by an air-drying mechanism and which are, by virtue of their predominantly hydrophilic character, suitable for application to cellulosic substrates, such as paper, in the form of an aqueous solution or emulsion.
The polymer compositions of the invention therefore are extremely useful as additives in the production of paper products where strength, absorbency, lightness and texture are important. It is not generally fully acceptable for most purposes if a paper product is very strong when dry but easily pulled apart when wet. A good balance of wet and dry strength is required. To obtain this good balance it is essential that the polymer before it is cured be dispersed in the paper web in such a way that it is capable of bonding a substantial number of the tangled fibers at the critical cross-over points where the fibers touch. This can be done by application of large amounts of the polymer, which may be uneconomic and could, especially if the polymer is essentially hydrophobic, affect adversely other desirable characteristics such as absorbency, handle, flexibility, softness and the like.
Alternatively a polymer can be used that is essentially hydrophilic such that it can be added as an aqueous solution or emulsion and will transport readily along the fibers and concentrate at the cross-over points before cross-linking. It is this form of polymer that is obviously the more efficient in developing wet strength.